In the control of systems having rotating drive shafts, the amount of torque applied to the drive shaft is an important parameter for control feedback. Therefore, the sensing and measurement of torque in an accurate, reliable and inexpensive manner has been a primary objective. For this purpose, non-contacting magnetoelastic torque transducers have been provided.
Magnetoelastic torque transducers commonly have two features—1) a shaft which is ferromagnetic and magnetostrictive; and 2) a means for detecting or sensing the measure of torque applied to the shaft. Ferromagnetism ensures the existence of magnetic domains within the shaft and magnetostriction allows the orientation of magnetization within each domain to be altered by the stress associated with applied torque.
Torque transducers based on the magnetoelastic response to torque induced mechanical stresses require an internal remanent magnetization of a controlled profile. One type of such transducer comprises a cylindrical shaft having bands of magnetization wherein the magnetization is circumferentially directed. The bands may be either a physically separate component applied to a shaft, e.g. a ring or collar affixed to the shaft to perform the active element function, or one or more magnetoelastic regions integrated into the axial length of the shaft.
Operation of a transducer for the measurement of torque applied to a shaft requires the shaft to be magnetically polarized in a substantially purely circumferential direction. A common method of magnetizing a transducer shaft includes the use of polarizing magnets to simultaneously create two magnetically contiguous polarized regions. The number of sources of polarizing fields will in general be the same as the number of polarized regions being created. The polarizing magnets are held close to the shaft surface while the shaft is rotated on its axis in either direction in the magnetic field produced externally to the shaft from the dipole-type magnetic source of the polarizing magnets. With this technique, it is difficult to control the magnetization profile. In addition, as a practical matter, it is extremely difficult to magnetize a shaft by conventional magnetization methods using polarizing magnets to a depth greater than about 1–2 mm because it is difficult to generate a strong enough magnetic field so far from the magnetic field source, due to the change in reluctance caused by the air gap between the magnet and shaft to be magnetized.
Moreover, the use of external polarizing magnets may result in uneven magnetization where the transducer material deepest within the shaft is insufficiently magnetized, leading to degraded transducer performance, such as reduced short term and long term sensitivity and the creation of “hot-spots”—nonuniformity in the transducer response. This technique is also difficult to optimize, configure and control.
With hollow shafts of large diameter, cooperating internal as well as external polarizing magnets also may be required to obtain a uniform, full-depth polarization of the active region(s), thereby increasing the cost of the apparatus.
An alternative method of magnetizing a shaft includes providing a current in an axial direction near the shaft, directly through the shaft or through a coaxial conductor passed through the central hole of the shaft. In torque transducers of the present invention where the active region is of generally limited axial extent and is to be located at some desirable axial position along the shaft, conventional methods involving the conduction of electrical currents through the entire shaft or through coaxial conductors passing through hollow shafts are unsuitable. Unlike conventional apparatus and methods, the apparatus and method of the present invention magnetizes a length of a shaft of limited axial extent in a substantially purely circumferential direction and throughout the entire depth or thickness of the length of the shaft or width of magnetic zone wanted.
Accordingly, there is a need for an improved apparatus and method to induce circumferential magnetization into a shaft.